In the building construction industry, the basic problems arising in the making of a flat wall, roof or floor are usually minimal so long as the surface is planar and uninterrupted. However, when it is necessary to provide an opening such as a scuttle or hatch, or the like, great care must be taken to ensure the tightness of any seams or joints between the opening and the planar surface. "Flashing" of various types and materials has been used, as disclosed and discussed in applicant's copending application Ser. No. 692,529 filed Jan. 18, 1985, now U.S. Pat. No. 4,603,517, which relates to an improvement in the frame corner flashing construction.